The invention relates generally to downhole driveshafts. More particularly, the invention relates to driveshafts employed in downhole motors used in drilling operations.
In drilling a borehole into an earthen formation, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of a drillstring formed from a plurality of pipe joints connected together end-to-end, and then rotate the drillstring so that the drill bit progresses downward into the earth to create a borehole along a predetermined trajectory. In directional drilling applications, specialized drillstring components and “bottom hole assemblies” (BHA) are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a borehole of desired non-vertical configuration. In addition, a typical BHA also includes a downhole or mud motor incorporated therein which further includes a power section, a bearing assembly disposed downhole of the power section, and a driveshaft assembly extending axially between the power section and bearing assembly. The power section is typically a progressive cavity or positive displacement motor (PD motor) that includes a rotor rotatably disposed within a stator.
In drilling operations, drilling fluid (e.g., drilling mud) is circulated under pressure through the drillstring and back up to the surface; however, en route to the drill bit, the pressurized drilling fluid flows through the power section, between the rotor and stator, to produce eccentric, precessional motion of the rotor with respect to the longitudinal axis of the stator, which thus generates rotational torque that is transferred through the driveshaft and bearing assemblies to rotate the drill bit about the central axis of the drillstring. During this process, the driveshaft assembly converts the eccentric motion of the rotor into rotary motion (i.e., concentric rotation) to rotate the bearing assembly and drill bit during drilling operations. For conventional driveshaft assemblies, such universal joints tend to wear or fail relatively quickly during operation. In particular, many conventional driveshafts transfer torque between various contact surfaces through either point or line contact(s), which disperse a large amount of force over a relatively small surface area, thereby tending to accelerate wear at such contact surfaces.